The present invention relates to novel photosensitive compositions and to photosensitive materials employing them. More particularly, it relates to compositions which are photohardenable by cationic polymerization.
Significant advances in UV curing based on the cationic polymerization of epoxy resins and in photoimaging technology based on photochemically induced acidolysis have been made following the discovery of thermally stable and highly efficient cationic photoinitiators such as diaryliodonium, triarylsulfonium and mixed-ligand arene cyclopentadienyl metal salts. Theoretically, diaryliodonium salts should have the highest efficiency among these initiators owing to their lower C-I bond energy and higher reactivity of dissociated species. However, the major absorption bands of these initiators fall at deep UV wave lengths (210-250 nm). The poor overlap of the absorption bands of iodonium salts with the emission spectra of commercially available medium and high-pressure mercury arc lamps has limited their efficiency and made them yield to thiophenoxy substituted triarylsulfonium salts in many important industrial applications. The latter have an additional absorption band at 300-400 nm and possess much higher efficiency.
Efforts have been made to extend the sensitivity of the initiators to longer wavelengths so that visible light could be used to initiate the cationic polymerization. Attempts have been made to synthesize new photoinitiators with longer wavelength absorption. It has been shown that the introduction of simple substitutes on the aryl rings does not markedly alter the spectral characteristics of iodonium salts. A diaryliodonium salt with a fluorenonyl group substituted on one aryl group has been synthesized. The formed compound has similar absorption and fluorescence spectra as fluorene and is the iodonium salt reported whose absorption wavelength goes to longest region. Despite its longer wavelength absorption, the rates of the polymerization initiated by this compound are not better than those of simple diaryliodonium photoinitiators.
Another technique for extending the photosensitivity of the photoinitiators to longer wavelength region is photosensitization. The photosensitization of iodonium salts by a wide variety of aromatic hydrocarbon, aromatic ketone, heterocyclic compounds and dyes has been studied. To some extent, these sensitizers can extend the sensitivity of iodonium salts to longer wavelengths, but few of them can provide visible light photoinitiation.
The photosensitization process can occur through three mechanisms: (1) energy transfer, (2) electron transfer and (3) free radical decomposition.
According to the energy transfer mechanism, an electronically excited photosensitizer is generated which interacts with the onium salt to promote it to its excited state while returning the photosensitizer to the ground state. Energy transfer photosensitization occurs for iodonium salts using photosensitizer with high triplet energy (&gt;70 kcal/mol) and high oxidation potentials which prevent competing electron transfer processes form taking place.
A triplet energy of 70 kcal/mole is equivalent to a photon of 400 nm wavelength. The sensitizer must absorb far below 400 nm irradiation in order to intersystem-cross to its triplet state, which means the sensitizer must also be a UV absorbing species and the sensitizer can not extend the sensitivity of the UV initiator to visible light region. It has also been confirmed that even if a triplet iodonium salt is formed, the excited molecule is still unreactive toward bond cleavage.
In the electron transfer photosensitization, an electron is transferred from the excited photosensitizer to the onium salt. The result is that a photoredox reaction occurs in which a photosensitizer is oxidized to a cation-radical while the onium salt is reduced to a radical species. In subsequent steps, the diaryliodide radical collapses to iodoaromatic compound and an aryl radical. This mechanism of photosensitization requires that the initiator of cationic polymerization be derived from the photosensitizer, while in the direct photolysis the initiator originates from fragments derived from the photosensitizer. The following compounds have been found to possess a sensitizing effect on iodonium salt initiated cationic polymerization:
______________________________________ Sensitizer Excitation energy (nm) (T*) ______________________________________ Anthracene 375 Perylene 432 Phenothiazine 500 Xanthone 385 Thioxanthone 432 ______________________________________
Thioxanthone(TX) is a typical example of these sensitizers. Substituted thioxanthones exhibit a strong absorption band in the region of 250-270 nm as well as a weak band at 380-420 nm in the UV-visible region. These spectral features make them very attractive as photosensitizers. Although strong interactions in the excited states have been observed between TX and diphenyliodonium (DPI) salts, the rates of the cationic polymerizations of difunctional epoxy monomers were little enhanced compared to the same polymerizations carried out in the absence of a photosensitizer.
In photosensitization by free radical induced decomposition, the primary photochemical process occurs on the photosensitizer. Bond dissociation or hydrogen abstraction of the excited photosensitizer produces radicals in solution. Oxidation of these radicals by DPI produces cations which initiate the cationic polymerization. Photosensitization occurs not as a result of a direct interaction between the onium salt and the excited photosensitizer but as a secondary "dark" nonphotochemical reaction of the onium salt with the radical products of the photosensitizer.
Summarizing, although many efforts have been made to develop a visible light cationic initiating system, no satisfactory system has been found. The most significant visible sensitizers developed to date are the five amino substituted dyes reported by Crivello and Lam, "Dye-sentized Photoinitiated Cationic Polymerization," Journal. of Polymer Science, Vol. 16, Polym. Chem. Ed., pp. 2441-2451 (1978).
U.S. Pat. No. 4,264,703 assigned to General Electric discloses that polymerization of a variety of polymerizable materials such as vinyl monomers, prepolymers, cyclic ethers, cyclic esters and cyclic organosiloxanes is effected by utilization of an initiator which includes aromatic halonium salt and, more particularly, diaryliodonium salts. These salts are described in greater detail in U.S. Pat. No. 4,264,703 and by Crivello and Lam, "Diaryliodonium Salts. A New Class of Photoinitiators for Cationic Polymerization," Macromolecules, Vol. 10 No. 6, pp. 1307-1315 (1977). An alternative class of cationic initiators is triarylsulfonium salts as disclosed in Crivello and Lam, "Photoinitiated Cationic Polymerization with Triarylsulfonium Salts," Journal of Polymer Science, Vol. 17, pp. 977-999 (1979). The cationic polymerizations described in the aforementioned references are initiated by exposure to ultraviolet radiation.
The practical application of the above onium salts as cationic photoinitiators has been limited by their low absorptivity at wavelengths above 300 nanometers. However, it has been discovered that the spectral response of onium salts may be extended into the near ultraviolet and visible wavelengths by energy transfer and electron transfer/redox sensitization. An extensive review of this extension is described in Crivello, Adv. in Polymer Sci., 62, 1 (1984). Crivello reports that through the use of certain photosensitizer-onium salt combinations, however, the process has tended to be extremely inefficient and hence of little practical utility for cationic photopolymerization.
Redox sensitization is known to occur by two distinct mechanisms. The first mechanism is direct electron transfer to release an acidic or cationic species. Examples of this mechanism have been described in U.S. Pat. No. 4,026,705 assigned to General Electric and in Crivello and Lam, "Dye-sensitized Photoinitiated Cationic Polymerization," Journal of Polymer Science, Vol. 16, Polym. Chem. Ed., pp. 2441-2451 (1978) and ibid. 17, p. 1059 (1979). As disclosed in these references an organic dye sensitizes the decomposition of onium salts, particularly diaryliodonium salts, upon exposure to radiation. Examples of dyes which are capable of photoexcitation as disclosed in the references include acridine and benzoflavin cationic dyes, benzophenone type basic dyes and perylene type neutral dyes. The quantum efficiency of the direct electron transfer is relatively low because there is a competing back electron transfer mechanism which reduces the amount of cationic species that are formed, and the spectral sensitivity has generally been limited to wavelengths less than 550 nm.
The second mechanism is indirect electron transfer wherein a free radical is photogenerated and subsequently undergoes electron transfer to the onium salt which releases an acid or cationic species. Examples of the indirect electron transfer mechanism are described in A. Ledwith, Polymer, 19, pp. 1217, 1219 (1978); Goetheis, ed. "Cationic Polymerization and Related Processes," Academic Press, p. 275 (1984); Crivello and Lee, Macromolecules, 14, 1141 (1981); Yagci et al., Makromol. Chem. Rapid Commun. 8, p. 209 (1987), and Makromol. Chem. Symp. 13 14, p. 161, (1988). Quantum yields greater than unity have been reported for the indirect electron transfer process. The photogenerated radicals from the sensitizer induce chain decomposition of onium salts and generate high yields of cations.
Few references describe or suggest the use of visible light free radical photoinitiators as the radical promoters for onium salt decomposition at wavelengths greater than 500 nm. Published European Application EP 0 408 227A1 to The Mead Corporation reports that certain dye-boranyl ion complexes can be combined with onium salts to provide viable cationic polymerization systems, throughout the visible spectrum. None of the references disclose or suggest the use of photoinitiators such as fluorones to extend initiation throughout the visible spectrum.
The above-described cationically polymerized materials have been used as moulding and extrusion resins, adhesives, caulks, coatings, printing inks, impregnated tapes, insulation, sealants, lubricants and the like.